This invention relates to an improved joint structure formed between components of which one is timber and which are secured by nail fasteners.
The invention, to be distinguished in our product range by the designation xe2x80x9cBlue Fastening Systemxe2x80x9d, has particular application to pallets, and is illustrated by that application. However, it is to be appreciated that the invention has application in joints other than those in pallets. The invention is concerned with providing a joint which is strengthened against forces acting parallel to a surface of each of the components at a join therebetween i.e. forces which place nails securing the joint in shear.
A well known type of timber pallet is of rectangular form in plan view and has a top, load supporting surface defined by a plurality of laterally extending, load support boards. The boards are secured in assembly by being nailed to laterally spaced side bearers each of which extends at right angles to and below a respective end of each board. Usually there is also an intermediate bearer to which the boards are nailed. The pallet can stand on the bearers but, more usually, a base is secured to the bearers.
In one form of pallet of that known type, herein referred to as xe2x80x9ca bearer palletxe2x80x9d, each of the bearers (including an intermediate bearer if provided) is in the form of an elongate beam which may be of substantially uniform cross-section or which may be notched. In such case the base usually comprises a plurality of laterally extending base boards, which may be similar to the support boards and which are nailed to each bearer. In another form of pallet of the known type, herein referred to as a xe2x80x9cblock palletxe2x80x9d, each of the bearers includes an elongate bearer plate and at least a respective bearer block below each end of the bearer plate, with the support plates nailed to the bearer plates and blocks. Usually there is an intermediate bearer block for each bearer plate. In the case of a block pallet, the base usually comprises a respective longitudinal base board below, and nailed to, the blocks of each side bearer plate and a lateral base board below and nailed to a respective block of each bearer plate.
With both bearer and block pallets, the pallet, and a load thereon, is able to be lifted by the tines of a fork lift device inserted into a cavity or a respective cavity defined between the side bearers, below the load support boards and, if provided, above the base. Damage to pallets is a relatively frequent occurrence, due to repeated impacts of the support or base boards by the tines in lifting the pallet, more specifically by the uprights from which the tines extend. Damage usually is to an end support board or a transverse end base board extending across the ends of the bearers, with damage being more frequent for end support boards than for end base boards by a factor of about 2 to 1. However, base boards also can be damaged by being dragged on hard surfaces such as concrete, while dragging can dislodge base boards.
Damage to the end support or base board necessitates pallets frequently needing to be taken out of service for repair. Eventually, the cumulative effect of damage and repair results in the need for a pallet to be discarded. Both repair, and replacement of a discarded pallet, represents a substantial cost factor. The damage frequently results from or involves movement of the end board relative to its bearers with a significant component parallel to the interface between the end board and bearers. This causes bending of the nails, to a somewhat S-shape which causes bruising of surrounding timber. Also, the bending tends to align the nails with the forces generating the movement, facilitating withdrawal of the nails from the end board or the bearers.
The present invention is concerned with providing an improved joint structure. In one preferred form, there is provided a pallet which has joints according to the invention and which, while not entirely eliminating the above cost factors, enables them to be reduced substantially. The pallet of the invention can be of the bearer form or of the block form.
A joint according to the invention is provided between two components of which at least one is of timber. The joint includes a nail plate provided between the components, and nails driven through one component and the nail plate, into the other component.
Each component may be of timber. Alternatively, one may be of timber and the other of another suitable material, such as a suitable grade of plastics material or light gauge steel plate. The components may comprise an end board and a bearer of a pallet, and reference to an end board and a bearer in the following description and claims is to be understood as illustrative of other components between which a joint according to the invention may be formed.
A pallet according to the invention has at least two laterally spaced bearers, and a plurality of laterally extending support boards extending over and secured to an upper surface of each bearer. The pallet also may include a base and, where this is the case, the base may include a plurality of laterally extending base boards extending under and secured to a lower surface of each bearer. As in the known forms of pallet, the support and, where provided, the base boards are in the form of elongate boards and, while successive support boards can abut, they usually are slightly spaced. However, a pallet according to the invention is strengthened, at a joint between at least one bearer and a lateral end board, by provision of a nail plate therebetween or within the bearer and by nails driven through the end board and nail plate, into the bearer. The end board is a support board or a base board which extends at ends or sides of the pallet.
Preferably a respective nail plate is provided between the lateral end board and each side bearer, to strengthen the pallet at each of two corners between which the end board extends. Where there is an intermediate bearer, a further nail plate can be provided between it and the lateral end board. The pallet can be further strengthened by a respective nail plate being provided between an end support board and the upper surface of each of the bearers, and between a base board and the lower surface of each of the bearers. In each case, the arrangement can be arranged at one end of the pallet or at each of its ends.
In the case of a bearer pallet, the nail plate of a join is provided between the end board and bearer. The same applies in the case of a block pallet where the end board is a base board, with the nail plate provided between the base board and a bearer block. However, where the end board is a support board of a block pallet, the or a respective nail plate of a join may be between the support board and the bearer plate of the bearer, and/or between the bearer plate and a block of the bearer.
In a joint according to the invention, the nail plate is substantially fixed relative to the one of the components of the joint in which the nail plate teeth are embedded. The portion of nails of the joint within the one component is substantially constrained against bending. Thus, with forces tending to cause relative movement between the components, parallel to the nail plate, bending of the nails is possible substantially only in the portion of each which is within the other component. As that portion of each nail is short relative to the overall nail length, the nails are substantially more resistant to bending under the action of a given force than would be the case if that force acted on the nails over their full length in the absence of a nail plate.
It is preferred that a respective nail plate be provided between each end board and each bearer. However, where a respective nail plate is to be provided between only one end board and each bearer, the end board preferably is a support board in the case of a bearer pallet and a base board in the case of a block pallet.
The nail plates, as will be appreciated, are formed from steel sheet metal from which a plurality of projecting teeth are punched, such as those available under the trade mark CLAW NAIL. The projecting teeth enable the plate to be applied to a surface of a timber member, or to surfaces of contiguous timber members, by being driven, by hand, hydraulically or pneumatically, to embed the teeth fully into the timber member or members. All teeth of a nail plate can extend in a common direction, from a main surface of the sheet metal, as is preferred for use in the present invention, or the nail plate can have teeth projecting from each of opposite main surfaces of the sheet metal.
The each nail plate preferably has an area which corresponds to a major part, most preferably substantially all, of the interface between the components of its joint, such as between a lateral board and bearer in a pallet joint. An interface area may, for example, be 150 mmxc3x9750 mm, and a suitable nail plate can have an area providing about 75% coverage of the interface area. That is the nail plate can be about 130 mmxc3x9745 mm. The nail plate can be from about 0.6 mm to 1.2 mm thick, such as from 0.8 to 1.0 mm thick.
A nail plate frequently has a uniform array of teeth over its surface area. However, those for use with the present invention preferably have at least one region from which teeth are not punched, to enable nails to be driven through clean metal of the nail plates. Such region preferably is centrally located, such as along a medial line, as appropriate for nails driven through an end board into a bearer of a pallet.
The tooth density of the nail plate, excluding such region from which teeth are not punched, may be from 0.8 to 1.5 teeth/cm2. The teeth preferably are of coined or twisted variety, most preferably with a respective tooth at each end of each of a plurality of slots formed on punching the teeth. The tooth length may be in the range of 8 to 13 mm, and preferably is 9 to 10 mm for hardwoods and 10 to 12 mm for use with softwoods.
In a pallet according to the present invention, each nail plate most preferably is applied to the upper or lower surface of a bearer, so that its teeth are embedded in the bearer. This is a most highly preferred arrangement. However, in a second possible arrangement, each nail plate can have teeth projecting from each of its main surfaces and be applied between the surface of a bearer and the adjacent surface of an end board, so that respective teeth are embedded in the bearer and the end board. In the second arrangement, the nail plate preferably has shorter teeth embedded in the end board and longer teeth embedded in the bearer. In a third arrangement, each nail plate is applied to and has its teeth embedded in a surface of an end board which is opposed to a bearer. In a further arrangement, in which the end board is a support board of a block pallet, the nail plate may be between the bearer plate and a block of a bearer, with its teeth embedded in either the bearer plate or the block, or with teeth of respective sets embedded in each of the bearer plate and block. However, not all of these arrangements may be available since a choice between them assumes that the bearers and also the end plates are of timber, as is preferred.
A pallet according to the invention need not be entirely of timber. Thus, while the support and base boards may be of timber, the bearers may be of a suitable plastics material or of light gauge steel. Alternatively, it is possible but less desirable for the bearers to be of timber and the support and/or base boards of plastics or light gauge steel. In these forms, each nail plate is able to have teeth projecting from only one of its major surfaces, with these being directed so as to become embedded in the timber component.
The spacing of the nails applied in each join can be closer than without the use of nail plates. Also, penetration of nails into a bearer and block may be approximately 10 times the nail diameter. The nails preferably are staggered. The width of the nails may be about three to four times the thickness of the plate from which the nail plates are formed. The nails may be driven by hand, hydraulically or pneumatically, with the upper limit for nail width being more appropriate where the nails are driven by hand or hydraulically rather than pneumatically.
Suitable nails can have a diamond point, a blunt diamond point or a blunt point. A diamond point is preferred as it penetrates the nail plate more satisfactorily. At least a majority of the nails most preferably pass through the nail plate predominantly, if not entirely, through clean metal of each nail plate, such as within a region from which teeth are not punched. Also, each nail preferably has its centre at two to three times the nail diameter from an edge of the nail plate, whether this be a peripheral edge or an edge of a slot from which teeth of the nail plate are punched.
The nails used to secure a joint according to the invention can take a variety of forms. In most forms, the nails in at least some longitudinal cross-sections include gripping means characterised by a longitudinal series of formations each of which tapers outwardly in a direction from its leading end to its head, such that the formations enable penetration, but act to resist withdrawal. The formations may be provided over substantially the full shank length of the nail. However, a portion of the shank length adjacent to the head of the nail may be free of the formations, with that portion having a length less than the thickness of the component, such as an end board, into which the nail is driven first.
The formations may be asymmetric in longitudinal section. Thus, each formation may have a shoulder which faces towards the head of the nail and which is at a greater angle to the shank than a surface providing the outward taper. The shoulder may be at an angle of up to about 90xc2x0 to the shank, or even in excess of 90xc2x0 so as to be re-entrant.
The formations evident in a longitudinal cross-section of the nails preferably are circumferentially continuous. In such case, they are of annular form and may be produced by rolling. Annular formations are preferred and contrary to expectations, are found to work well with hardwood components. However, the formations need not be circumferentially continuous. In the latter case, the formations may be in annular bands, such as would result from providing longitudinal grooves along the shank of a nail having annular or helical formations. However, the formations can be of a form resulting from other procedures in which they are upset from the shank of a suitable nail.
The provisions of a respective nail plate at corner joints between which an end board extends, and at intermediate joints at intermediate bearers, provides for substantial strengthening of the joints and hence, resistance of the pallet to damage. As a consequence, the average frequency with which a pallet needs to be taken out of service, such as for replacement of an end board, can be substantially reduced. That is, the average time between successive repair intervals for a pallet can be substantially increased. In each case, the improvement is relative to pallets of the known form which do not have nail plates and gripping means as required by the invention.
The maximum benefit is obtained with pallets according to a preferred arrangement, having nail plates with teeth embedded only in the upper and/or lower surface of the bearers, and the end boards secured by nails having annular formations applied through the end boards and nail plates so as to extend into the bearers. In comparative testing between such pallets and pallets of the known form, up to a ten-fold increase in the average time between the need for repair of the pallets was obtainable with pallets according to the invention, relative to the average time required for the known pallets. However, substantial benefits also are obtained with pallets having a respective set of teeth embedded in each of a bearer and end board, or with teeth embedded only in an end board.
In the comparative testing, samples of the two types of pallets were subjected to repeated blows simulating impact by the tines of a lifting device such as experience in normal use. The pallets according to the preferred arrangement of the invention were found to have greatly enhanced corner joint strength and impact resistance for their end boards, due to the joints restricting the tendency for blows against the end boards to develop a detrimental level of vertical force components in the nails. This is attributed to the nails operating substantially in shear, due to their interaction with the nail plates. The nails are found to provide a tight fit between their gripping means and the nail plates, achieved by the nails bursting through the nail plates as they are applied to form holes through the nail plate somewhat ballistically. At least during initial impacts, the gripping means are set against the edge of holes of the nail plates to achieve a positive lock therebetween.
The positive locking is best achieved by nails having gripping means comprising a longitudinal series of continuous annular formations of longitudinally asymmetric form. However, improved results can be achieved with nails having gripping means comprising discontinuous annular arrays of formations of longitudinally asymmetric form and to a lesser extend, nails having gripping means comprising discontinuous helical arrays of formations of longitudinally asymmetric form. Improved results also can be achieved with nails having gripping means comprising continuous helical thread of longitudinally asymmetric form, but this form generally is less desirable than the others and, in particular, is less desirable than gripping means of continuous annular formations.
In each case, the reference to asymmetric form denotes the form in longitudinally section. The formations, in such section, have respective shoulders facing towards and away from the head of the nail, with the shoulder acing the head being at a greater angle to the axis of the nail than the other shoulder. In each case, the positive locking provided by a formation is achieved by its shoulder facing the head.
The improved joints achieved in pallets according to the invention result from the combined action of the nail plates and nails having suitable gripping means. The use of nail plates with conventional nails, having a substantially uniform shank cross-section, provides lesser improvement over use of such nails without the nail plates. The use of nails with suitable gripping means, but without use of nail plates, also provides little improvement. In each case, impacts generate force components tending to cause withdrawal of the nails and failure of the joint. With use of conventional nails with nail plates, holes punched in the latter during application of the nails provide less resistance to withdrawal. With nails having gripping means, but used without nail plates, the initial response of the nails to impact is to bruise surrounding timber and to align themselves with force components tending to cause their withdrawal, with ultimate withdrawal of the nails and failure of the joint.
When a pallet according to the invention does necessitate repair, the nature of the repair can vary with the extent of damage. In some instances, it can be sufficient simply to clamp or press an end board back into position and drive in a sufficient number of further nails with gripping means. Where the damage is more extensive, an end board may need to be replaced, generally with forced withdrawal or cutting of the nails for that board, and positioning a new end board and securing this by fresh nails having gripping means.